1. Field of the Invention
This invention relates generally to wireless communications and, more particularly, to the operation of a Radio Frequency (RF) transceiver within a component of a wireless communication system.
2. Description of the Related Art
The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e., operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, Medium Access Control (MAC) layer operations, link layer operations, etc. By complying with these operating standards, equipment interoperability is achieved.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that may be caused by transmissions on a particular channel.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all, cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include RF transceivers. Radio frequency transceivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. Similarly, the RF receiver receives an RF signal, down converts it to IF and then to baseband. In other systems, the received RF is converted directly to baseband.
In down converting a signal (either an IF or RF signal) to a baseband frequency signal, the signal is mixed with a reference signal having a specified frequency that is received from a local oscillator (LO). As used herein, “local oscillator” is a device that provides a fixed frequency to a mixer that is to be mixed with a signal of interest, e.g., RF signal or IF signal. Because the mixer's ability to accurately down convert (or up convert for a transmitter stage) a signal depends upon it receiving an accurate frequency signal from the local oscillator, many local oscillators are formed to be adjustable so as to adjust an output frequency to a number of supported RF channels and to account for variations due to temperature, process, manufacturing and other factors that may affect the precise frequency that is produced by the local oscillator.
The reference signal used by the mixer is often provided by a phase-locked loop that includes a charge pump (circuit for sinking or sourcing a current), a loop filter (a low pass filter) and a voltage-controlled oscillator (to provide a signal with frequency that is a function of an input voltage level). A function of the charge pump is to source current into or sink current out of a loop filter that is coupled between the charge pump and the voltage-controlled oscillator of the phase-locked loop system. Typical charge pump designs include a current sink and a current source that are selectively coupled to an output of the charge pump so that, based upon appropriate control signals, current may be sinked or sourced as necessary to lower or increase a voltage applied to the voltage-controlled oscillator. Known charge pump designs do not, however, provide for current sinking or sourcing in a precise manner, especially when the charge pump requires very little adjustment for providing a desired amount of current to a loop filter and, therefore, voltage to an input of a voltage-controlled oscillator (VCO).
What is needed is a charge pump that provides more precise current sinking and sourcing from/to a loop filter and VCO.